Hand and foot movements are made more reliably when both limbs move in the same direction at the same time (isodirectional) compared with when they are made in opposite directions (anisodirectional). We hypothesized that M1 intracortical facilitation may subserve hand-foot coordination and reveal correlates that explain the preference for hand-foot movements to be performed in an isodirectional pattern. To test our hypothesis we investigated behavioral kinematics of hand-foot coordination (experiment 1) and neurophysiological measures of corticomotor excitability and intracortical facilitation (experiment 2) in 17 healthy young adults. As expected, coordination became unstable in the anisodirectional pattern but not the isodirectional pattern, as confirmed in measures of wrist and ankle relative phase error and stability (both P < 0.001). Short-latency paired-pulse TMS was used to elicit motor evoked potentials (MEPs) and produce short-latency intracortical facilitation (sICF) in right extensor carpi radialis (ECR) and flexor carpi radialis (FCR) in the presence and absence of right ankle plantarflexion/dorsiflexion (P < 0.015). An isodirectional preference was confirmed by facilitation of FCR MEPs and TMS-induced wrist flexion during ankle plantarflexion (both P < 0.025) but no evidence of modulation of any particular "I wave" during foot movement compared with rest. A novel finding was the association between loss of stability of the anisodirectional pattern (experiment 1) and the modulation of corticomotor excitability in support of the isodirectional pattern (experiment 2) (P < 0.05). The preference for isodirectional hand-foot movements appears not to depend on M1 intracortical facilitation.
Keywords: TMS; interlimb coordination; intracortical facilitation; primary motor cortex.